Optical equalizers are often used in optical wavelength division multiplexing (WDM) networks. The term “optical equalizer” simply refers to a device that receives light of multiple wavelengths and couples at least some fraction of the received light of at least one of the wavelengths out of the device in order to equalize the fractions of light of the different wavelengths remaining in the device. This equalization of fractions of light of different wavelengths is typically referred to as power equalization of optical channels, where each channel corresponds to a different wavelength of light.
Some of the functions performed by optical equalizers in WDM networks include removal of residual gain ripples in optical amplifiers and equalization of signal levels among WDM channels to optimize signal-to-noise ratio (SNR). Existing equalizer devices that have been used to perform these functions include Mach-Zender or acoustic-optical filters, holograms and MicroElectroMechanical System (MEMS) mirrors. These devices tend to operate over a large spectral range and do not provide for channel-to-channel equalization. Optical apparatuses that provide channel-to-channel equalization are typically complicated in structure and require a multiplexer, a demultiplexer and an array of equalizer devices. The demultiplexer device separates the different wavelengths of light to enable each wavelength of light to be sent to a respective equalizer device of the array of equalizer devices. The multiplexer device re-combines the equalized wavelengths of light output from the equalizer devices. The multiplexer and demultiplexer are sources of additional optical signal loss in the system, which is undesirable. Furthermore, a separate equalizer device is required for each channel being handled by the apparatus.
Accordingly, a need exists for an equalizer device that is capable of performing channel-to-channel equalization, that is relatively simple in structure, and that overcomes the disadvantages of existing optical equalizer devices, such as the necessity of using a multiplexer/demultiplexer and an array of equalizer devices to perform channel-to-channel equalization, as well as the optical losses associated with such configurations.